fix(serve): keep json_logging's startup debug line out of plain-text stderr

[vinismarques]

Summary

With ENABLE_JSON_LOGGING=true, server startup wrote one plain-text line to stderr:

DEBUG:json_logging:Update all existing logger to using JSONLogFormatter

Log collectors that parse stderr as JSON (e.g. Google Cloud Logging) record any non-JSON stderr line as ERROR, so every start showed up as a spurious error.

import json_logging
  get_library_logger("json_logging")   # pins the library logger to DEBUG
logging.basicConfig(...)               # root handler: plain text on stderr
json_logging.init_fastapi(...)
  _logger.debug("Update all existing logger ...")   # → plain line via root
json_logging.config_root_logger()      # root handler switches to JSON, too late

 if settings.enable_json_logging:
     register_client_safe_request_extractor()
+    logging.getLogger("json_logging").setLevel(logging.INFO)
     json_logging.init_fastapi(enable_json=True)

Reordering basicConfig before the import doesn't fix this: the library always pins its logger to DEBUG on first import. Its WARNING and ERROR messages still get through.

Evidence

• Before: test_json_logging_startup_writes_only_json (start the app in a fresh interpreter, assert every stdout/stderr line parses as JSON) fails:
  JSONDecodeError on 'DEBUG:json_logging:Update all existing logger to using JSONLogFormatter'
  After: passes. test_json_logging_keeps_reporting_its_own_warnings confirms a WARNING from the json_logging logger is still emitted as JSON.

Merge Danger

Door: two-way

Blast Radius: cosmetic

Only DEBUG records from the json_logging logger are dropped. Side effect: setting the library's own ENABLE_JSON_LOGGING_DEBUG no longer surfaces its DEBUG lines (its INFO and WARNING ones still appear).

Bumps the version to 5.2.1 for the release.